Method and a device for manufacturing an optical preform by means of an internal vapour deposition process, as well as corresponding substrate tube assembly

ABSTRACT

A device for manufacturing an optical preform by means of an internal vapor deposition process including an energy source, a hollow substrate tube having a supply side and a discharge side and the energy source being moveable along a length of the hollow substrate tube, and an elongation tube connected to the hollow substrate tube at the discharge side thereof, wherein the hollow substrate tube extends into an interior of the elongation tube and an internal diameter of the elongation tube is at least 0.5 millimeters larger than an external diameter of the hollow substrate tube.

TECHNICAL FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a device for manufacturing an opticalpreform by means of an internal vapour deposition process, the devicecomprising an energy source and a hollow substrate tube, wherein thehollow substrate tube has a supply side and a discharge side, the energysource being moveable along a length of the hollow substrate tube, thedevice further comprising an elongation tube connected to the hollowsubstrate tube at the discharge side thereof.

Generally, in the field of optical fibres, multiple thin films of glassare deposited on the inside surface of a substrate tube. The substratetube is hollow to allow internal deposition. The substrate tube may beof glass, for example glass quartz (SiO₂). Glass-forming gases (viz.reactive gases comprising gasses for the forming of glass and optionallyprecursors to dopants) are introduced into the interior of the substratetube from one end (called the “supply side” of the substrate tube).

Doped or undoped glass layers (depending on the use of reactive gaseswith or without one or more precursors to dopants, respectively) aredeposited onto the interior surface of the substrate tube. The remaininggases are discharged or removed from the other end of the substrate tubecalled the “discharge side” of the substrate tube. The removal isoptionally carried out by means of a vacuum pump. The vacuum pump hasthe effect of generating a reduced pressure in the interior of thesubstrate tube, which reduced pressure generally comprises a pressurevalue ranging between 5 and 50 mbar, i.e. 500 and 5000 Pascal.

Several types of internal chemical vapour depositions (CVD) are known,vapour axial deposition (VAD), modified chemical vapour deposition(MDVD) and plasma-enhanced chemical vapour deposition (PECVD or PCVD).Plasma-enhanced chemical vapour deposition (PECVD or PCVD) is a processused to deposit thin films from a gas state (vapour) to a solid state ona substrate. Chemical reactions are involved in the process, which occurafter creation of a plasma of the reacting gasses.

Generally, the plasma is induced by the use of electromagneticradiation, preferably microwaves. Usually, electromagnetic radiationfrom a generator are directed towards an applicator via a waveguide,which applicator surrounds the substrate tube. The applicator couplesthe electromagnetic radiation into a plasma that is generated inside thesubstrate tube. The applicator is moved reciprocally in the longitudinaldirection of the substrate tube. Thus, the plasma formed, also calledthe “plasma reaction zone”, is also moved reciprocally. As a result ofthis movement a thin vitrified silica layer is deposited onto theinterior of the substrate tube with every stroke or pass.

The applicator and the substrate tube are generally surrounded by afurnace so as to maintain the substrate tube at a temperature of900-1300° C. during the deposition process.

Thus, the applicator is moved in translation over the length of thesubstrate tube within the boundaries of a furnace which surrounds thesubstrate tube and the applicator reciprocating within the furnace. Withthis translational movement of the applicator the plasma also moves inthe same direction. As the applicator reaches the inner wall of thefurnace near one end of the substrate tube, the movement of theapplicator is reversed so that it moves to the other end of thesubstrate tube towards the other inner wall of the furnace. In otherwords the applicator and thus the plasma is reciprocating between areversal point at the supply side and a reversal point at the dischargeside of the substrate tube. The applicator and thus the plasma travels aback and forth movement along the length of the substrate tube. Eachback and forth movement is call a “pass” or “stroke”. With each pass athin layer of vitrified silica material is deposited on the inside ofthe substrate tube.

Normally, a plasma is generated only in a part of the substrate tube,viz. the part that is surrounded by the applicator. The dimensions ofthe applicator are smaller than the dimensions of the furnace and of thesubstrate tube. Only at the position of the plasma, the reactive gassesare converted into solid glass and deposited on the inside surface ofthe substrate tube. Since the plasma reaction zone moves along thelength of the substrate tube, glass is deposited more or less evenlyalong the length of the substrate tube.

When the number of passes increases the cumulative thickness of thesethin films, i.e. of the deposited material, increases thus leading to adecrease in the remaining internal diameter of the substrate tube. Inother words, the hollow space inside the substrate tube keeps gettingsmaller with each pass.

During the deposition process, the substrate tube is clamped into aglass worker lathe. The applicator moves reciprocally only over a partof said substrate tube. This has the disadvantage that only a part ofsaid, expensive, substrate tube can be used to prepare optical fibers.In order to overcome said problem, it is know e.g. from the publicationsbelow, to attach a piece of lower quality glass tube, e.g. a so-calledelongation tube, to at least the discharge side of said substrate tube.This elongates the total length of the tube. The elongation tubes areclamped into the glass working lathe which increases the effectivelength of the substrate tube that can be used for deposition.

From European patent application EP 1,801,081 in the name of the presentapplicant, a device is disclosed for manufacturing an optical preform bymeans of an internal vapour deposition process, wherein an insertiontube is present in the interior of the substrate tube, at the dischargeside, wherein the external diameter and the shape of the insertion tubesubstantially correspond to the internal diameter and the shape of thesubstrate tube, and wherein the insertion tube extends beyond thesubstrate tube. In other words, the insertion tube is inserted in theend of the substrate tube.

From Japanese patent application JP 2003-176148 a method ofmanufacturing a preform of an optical fibre is known, comprisingcoaxially attaching an exhaust tube to a quartz tube.

From U.S. Pat. No. 4,389,229 a method of fabricating a light guidepreform by a modified chemical vapour deposition process is known,wherein undeposited reactants pass through a glass substrate tube andflow into a reactant exhaust system and are carried therethrough by auniformly flowing reactant-free gas. The reactants pass through anexhaust tube, a reactant collection chamber, through a pressure controlapparatus and into a gas scrubber. The pressure within the exhaustsystem is maintained substantially constant during the process bycontinuously monitoring the pressure therein and adjusting the pressurecontrol apparatus accordingly.

From European patent application EP 1,988,062 a device and a method formanufacturing an optical preform by means of an internal vapourdeposition process are known, comprising an energy source and asubstrate tube, which substrate tube has a supply side for supplyingglass-forming precursors and a discharge side for discharging componentswhich have not been deposited on the interior of the substrate tube,whilst the energy source is movable along the length of the substratetube between a reversal point on the supply side and a reversal point onthe discharge side.

One drawback of, for example, the Japanese patent application JP2003-176148 is that glassy material deposited outside the depositionarea in an internal vapour deposition process gives raise to mechanicalstress build-up in the substrate tube. This mechanical stress may leadto the substrate tube to break during the optical preform production,which is undesirable.

Another drawback of the known devices comprising elongation tubesattached to the discharge side of the substrate tube is that theconnection between the elongation tube and the hollow substrate tube issubjected to a mechanical tension during the following collapsing stepwhich might result in cracking of the substrate tube or resultingprimary preform which is undesirable.

In the prior art there is additional problem that may lead to crackingof the substrate tube or primary preform, being the presence of sootinside of the elongation tube. When the internal deposition process iscompleted the substrate tube having deposited layers of glass on theinside surface thereof is removed, often still having a very hightemperature, such as e.g. 800-900 degrees Celsius. When this substratetube is then slightly tilted, a so-called chimney effect arises causingpart of the soot to flow into the substrate tube causing pollution ofthe glass layers. When the soot is not removed, it might cause crackingduring the collapsing process due to mechanical stress applied to saidelongation tube. This can be overcome by manually removing said sootfrom said elongation tube prior to tilting it, but this is difficult todo due to the high temperature.

This problem has previously been solved in the prior art by introducingan insertion tube into the substrate tube. This insertion tube “catches”the soot and can be easily removed from the substrate tube before thesubsequent collapsing.

A drawback of the known devices in which an insertion tube is insertedinto the substrate tube is that this leads to the built up of undesiredglassy deposition on the inner surface of the substrate on thelongitudinal position adjacent the insertion tube which glassydeposition leads to an increase in crack formation. This will beexplained in more detail below.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide for a device formanufacturing an optical preform by means of an internal vapourdeposition process in which the internal stress build-up in the glasscaused by glasses deposited just outside the deposition area, as well asthe mechanical tension of the connection between the hollow insertiontube and the hollow substrate tube is reduced.

It is another object of the present invention to provide for a methodfor manufacturing an optical preform by means of an internal vapourdeposition process, in which the above mentioned internal stressbuild-up and the mechanical tension is avoided, or at least reduced.

It is another objection of the present invention to provide a substratetube assembly without the drawbacks of the prior art.

The above mentioned objects are achieved by the present invention.

The present invention relates in a first aspect to a device, in a secondaspect to a method, and in the third aspect to a substrate tubeassembly.

In said first aspect, the present invention relates to a device formanufacturing an optical preform by means of an internal vapourdeposition process, the device comprising an energy source and a hollowsubstrate tube, which hollow substrate tube has a supply side and adischarge side, the energy source being moveable along a length of thehollow substrate tube, the device further comprising an elongation tubeconnected to the hollow substrate tube at the discharge side thereof,wherein in that the hollow substrate tube extends into an interior ofthe elongation tube and in that an internal diameter of the elongationtube is at least 0.5 millimeters larger than an external diameter of thehollow substrate tube.

In an embodiment of said aspect, the hollow substrate tube extends intothe interior of said elongation tube with a length between 0.5 and 10centimeters.

In an embodiment of said aspect, the hollow substrate tube extends intothe interior of said elongation tube with a length between 2 and 5centimeters.

In an embodiment of said aspect, the hollow substrate tube extends intothe interior of said elongation tube with a length between 2.5 and 3.5centimeters.

In another embodiment of said aspect, the internal diameter of theelongation tube is between 0.5 and 5 millimeters larger than theexternal diameter of the hollow substrate tube.

In another embodiment of said aspect, the internal diameter of theelongation tube is between 1 and 2 millimeters larger than the externaldiameter of the hollow substrate tube.

In another embodiment of said aspect, the elongation tube comprises ahollow insertion tube.

In another embodiment of said aspect, said hollow insertion tube isplaced coaxially with respect to said elongation tube.

In another embodiment of said aspect, said insertion tube is oriented inline with the hollow substrate tube.

In another embodiment of said aspect, said insertion tube is mounted, inline with said hollow substrate tube.

In another embodiment of said aspect, the insertion tube is orientedsuch that a spacing between one end of the hollow insertion tube and oneend of the hollow substrate tube that extends into the interior of saidelongation tube is at least 0.5 millimeters, preferably at least 2millimeters.

In another embodiment of said aspect, the internal diameter of thehollow insertion tube is at least equal to the internal diameter of thehollow substrate tube.

In another embodiment of said aspect, the elongation tube is connectedto the hollow substrate tube at one end of the elongation tube.

In said second aspect, the present invention relates to a method formanufacturing a precursor for a primary preform by means of an internalvapour deposition process, wherein glass-forming precursors are suppliedto a hollow substrate tube on a supply side thereof, which substratetube further has a discharge side, wherein an energy source is movedalong a length of the hollow substrate tube for the purpose ofgenerating deposition conditions in the hollow substrate tube, andwherein a plurality of glass layers is deposited on the inner surface ofsaid substrate tube to form said precursor for a primary preform,wherein an elongation tube is mounted over the hollow substrate tube, atthe discharge side thereof, such that the hollow substrate tube extendsinto an interior of the elongation tube, and in that an internaldiameter of the elongation tube is at least 0.5 millimeters larger thanan external diameter of the hollow substrate tube.

In an embodiment of said aspect, the hollow substrate tube extends intothe interior of said elongation tube with a length between 0.5 and 10centimeters. In other words, the hollow substrate tube and theelongation tube are positioned with respect to each other in such amanner that the hollow substrate tube extends into the interior of saidelongation tube with a length between 0.5 and 10 centimeters.

In an embodiment of said aspect, the hollow substrate tube extends intothe interior of said elongation tube with a length between 2 and 5centimeters. In other words, the hollow substrate tube and theelongation tube are positioned with respect to each other in such amanner that the hollow substrate tube extends into the interior of saidelongation tube with a length between 2 and 5 centimeters.

In an embodiment of said aspect, the hollow substrate tube extends intothe interior of said elongation tube with a length between 2.5 and 3.5centimeters. In other words, the hollow substrate tube and theelongation tube are positioned with respect to each other in such amanner that the hollow substrate tube extends into the interior of saidelongation tube with a length between 2.5 and 3.5 centimeters.

In an embodiment of said aspect, the internal diameter of the elongationtube is between 0.5 and 5 millimeters larger than the external diameterof the hollow substrate tube. In other words, the hollow substrate tubeand the elongation tube are positioned with respect to each other insuch a manner that the internal diameter of the elongation tube isbetween 0.5 and 5 millimeters larger than the external diameter of thehollow substrate tube.

In an embodiment of said aspect, the internal diameter of the elongationtube is between 1 and 2 millimeters, larger than the external diameterof the hollow substrate tube. In other words, the hollow substrate tubeand the elongation tube are positioned with respect to each other insuch a manner that the internal diameter of the elongation tube isbetween 1 and 2 millimeters larger than the external diameter of thehollow substrate tube.

In an embodiment of said aspect, the elongation tube comprises a hollowinsertion tube oriented and mounted in line with the hollow substratetube such that a spacing between one end of the hollow insertion tubeand one end of the hollow substrate tube that extends into the interiorof said elongation tube is at least 0.5 millimeters.

In an embodiment of said aspect, the elongation tube comprises a hollowinsertion tube oriented and mounted in line with the hollow substratetube such that a spacing between one end of the hollow insertion tubeand one end of the hollow substrate tube that extends into the interiorof said elongation tube is at least 2 millimeters.

In an embodiment of said aspect, an internal diameter of the hollowinsertion tube is at least equal to an internal diameter of the hollowsubstrate tube.

In an embodiment of said aspect, the elongation tube is connected to thehollow substrate tube at one end of the elongation tube.

In said third aspect, the present invention relates to a substrate tubeassembly for use in an internal vapour deposition process, the substratetube assembly comprising a hollow substrate tube having a supply sideand a discharge side wherein the substrate tube assembly comprises anelongation tube mounted over the hollow substrate tube, at the dischargeside thereof, wherein the hollow substrate tube extends into an interiorof the elongation tube, and in that an internal diameter of theelongation tube is at least 0.5 millimeters larger than an externaldiameter of the hollow substrate tube.

In an embodiment of said aspect, the hollow substrate tube extends intothe interior of said elongation tube with a length between 0.5 and 10centimeters. In an embodiment of said aspect, the hollow substrate tubeextends into the interior of said elongation tube with a length between2 and 5 centimeters.

In an embodiment of said aspect, the hollow substrate tube extends intothe interior of said elongation tube with a length between 2.5 and 3.5centimeters.

In an embodiment of said aspect, the internal diameter of the elongationtube is between 0.5 and 5 millimeters larger than the external diameterof the hollow substrate tube.

In an embodiment of said aspect, the internal diameter of the elongationtube is between 1 and 2 millimeters larger than the external diameter ofthe hollow substrate tube.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention are better understood when the following detailed descriptionof the invention is read with reference to the accompanying drawings, inwhich:

FIG. 1 illustrates a first example of a substrate tube assemblyaccording to the present invention;

FIG. 2 illustrates a second example of a substrate tube assemblyaccording to the present invention; and

FIG. 3 illustrates a third example of a substrate tube assemblyaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions are used in the present description and/orclaims to define the stated subject matter. Other terms not cited beloware meant to have the generally accepted meaning in the field.

“Hollow substrate tube” as used in the present description means: anelongation tube having a cavity within. Generally, the inside of saidtube is provided (or coated) with a plurality of glass layers during themanufacturing of a preform.

“Gas supply side” or “supply side” as used in the present descriptionmeans: one side of the substrate tube, being an open end of thesubstrate tube that is used as inlet for the gases. The supply side isthe side opposite to the discharge side.

“Gas discharge side” or “discharge side” as used in the presentdescription means: one side of the substrate tube, being an open end ofthe substrate tube that is used as outlet for the gases. The dischargeside is the side opposite to the supply side.

“Interior surface” as used in the present description means: the insidesurface or inner surface of the hollow substrate tube.

“Glass” or “glass material” as used in the present description means:crystalline or vitreous (glassy) oxide material—e.g. silica (SiO₂) oreven quartz—deposited by means of a vapour deposition process.

“Silica” as used in the present description means: any substance in theform of SiOx, whether or not stoichiometric, and whether or notcrystalline or amorphous.

“Glass-forming gases” as used in the present description means: reactivegases used during the deposition process to form glass layers. Theseglass forming gases may comprise a precursor for a dopant. (e.g. O₂ andSiCl₄ and optionally others).

“Reaction zone” as used in the present description means: the zone oraxial location wherein the glass-forming reaction or deposition takesplace. This zone is formed by a plasma and preferably moves reciprocallyalong the longitudinal length of the substrate tube.

“Plasma” as used in the present description means: an ionized gasconsisting of positive ions and free electrons in proportions resultingin more or less no overall electric charge at very high temperatures.The plasma is usually induced by microwaves.

“Reversal point” as used in the present description means: the axialpoint or position on the substrate tube at which the movement of theapplicator reciprocates. In other words, changes from back to forth andforth to back. It is the turning point of the applicator. The axialpoint is measured at the middle (longitudinal) of the applicator.

“Near the reversal point” as used in the present description means: anaxial position on the substrate tube that is close in distance to thereversal point, or is the same position as the reversal point.

“At the reversal point” as used in the present description means: anaxial position on the substrate tube that is the same position as thereversal point.

“Moved back and forth” as used in the present description means: areciprocating movement or moving backwards and forwards in a straightline.

“Stroke” or “pass” as used in the present description means: each backand forth movement of the applicator along the length of the substratetube.

“Elongation tube” as used in the present description means: a glass tubethat is attached to at least the discharge side of the substrate tube inorder to elongate the substrate tube.

“Insertion tube” as used in the present description means: a glass tubethat is inserted into the substrate tube and/or the elongation tube. Theabove-mentioned and other features and advantages of the invention willbe best understood from the following description referring to theattached drawings. In the drawings, like reference numerals denoteidentical parts or parts performing an identical or comparable functionor operation.

The invention is not limited to the particular examples disclosed belowin connection with a device for manufacturing an optical preform or aparticular method for manufacturing the optical preform.

The present invention relates, in a first aspect, to a device formanufacturing an optical preform by means of an internal vapourdeposition process, the device comprising an energy source and a hollowsubstrate tube, wherein the hollow substrate tube has a supply side anda discharge side, the energy source being moveable along a length of thehollow substrate tube, the device further comprising an elongation tubeconnected, in a substantially air-tight manner, to the hollow substratetube at the discharge side thereof, wherein the hollow substrate tubeextends into an interior of the elongation tube, thereby defining anoverlapping region between the hollow substrate tube and the elongationtube, and in that an internal diameter of the elongation tube at theoverlapping region is at least 0.5 millimeters larger than an externaldiameter of the hollow substrate tube at the overlapping region.

In other words, the elongation tube is provided coaxially over one endof the substrate tube.

The invention is based on the finding by the inventors that, in case thehollow substrate tube extends into the interior of the elongation tube,the mechanical stress build-up may be reduced. The inventors have foundthat the mechanical stress build-up may be caused by the following.

The inventors noted that the mechanical tension between the elongationtube and the hollow substrate tube is avoided, or at least reduced, incase the hollow substrate tube extends into the interior of theelongation tube. This, due to the fact that the elongation tube may beconnected to the outer surface of the hollow substrate tube, along thecircumference thereof spatially distant by e.g. at least 2.5 centimetersfrom the end face of the substrate tube, instead of a connection of theend face of the elongation tube to the end face of the hollow substratetube. This provides for a better rigid connection between these tubes.

In the context of the present invention, the energy source may be atraversing applicator moveable along the length of the substrate tubebetween a point of reversal at the supply side thereof and a point ofreversal at the discharge side thereof for generating depositionconditions inside the hollow substrate tube.

In an embodiment, the elongation tube is fused to the hollow substratetube.

In an embodiment, the hollow substrate tube extends into the interior ofthe elongation tube with a length between 0.5 and 10 centimeters,preferably between 2 and 5 centimeters, even more preferably between 2.5and 3.5 centimeters.

It is noted that in case the length of the overlapping region isapproximately of the above mentioned length, the connection between theelongation tube and the hollow substrate tube becomes more rigid, i.e.more reliable. This provides less change of breaking or cracking of thepreform.

In another embodiment, the internal diameter of the elongation tube atthe overlapping region is between 0.5 and 5 millimeters, preferablybetween 1-2 millimeters, larger than the external diameter of the hollowsubstrate tube at the overlapping region.

An advantage of this embodiment is that inner diameter of the elongationtube does not have physical contact with the outer surface of thesubstrate tube at the end face thereof. Since the end face of thesubstrate tube is spatially distant from the inner surface of thesubstrate tube, there is no continuation of glass deposit. In otherwords, glass deposited in the elongation tube is not in (physical)connection with the glass deposited in the interior of the hollowsubstrate tube. Any mechanical tensions or the like occurring in theglass deposited on the elongation tube will not propagate to the glassdeposited in the interior of the hollow substrate tube. This preservesthe quality of the substrate tube.

Another advantage is that the joining position, i.e. the position atwhich the elongation tube connected to the hollow substrate tube, isshifted upwards of the hollow substrate tube resulting in an even morerigid connection between these tubes.

Yet another advantage of this embodiment is that the spacing between theelongation tube and the hollow substrate tube provides the possibilitythat an insertion tube may be accurately mounted in line with the hollowsubstrate tube, as, for example, the diameter of the insertion tube maybe matched to the diameter of the hollow substrate tube. This relates toan embodiment discussed in more detail below.

Further, according to the present invention, an insertion tube may beused for draining or discharging gasses not deposited in the hollowsubstrate tube. Moreover, the insertion tube is used to “catch” anyadditional deposition outside of the heat source region. After thedeposition process is finished, the insertion tube is removed from theelongation tube and together with this the deposition of unwanted glassis removed. The substrate tube and attached elongation tube is thenready for the next step in the process, being the collapsing step.

The present inventors have found that the presence of an insertion tubeinserted into the end of the substrate tube may lead to the built up ofaddition, internal stress in addition to the mechanical stress discussedabove. The present inventors believe that this stress built up is theresult of two points.

The first point is related to turbulence of the glass-forming gassesexiting the hollow substrate tube and entering the hollow insertiontube. In case that the insertion tube is inserted into the end of thesubstrate tube, for example, the diameter of the hollow insertion tubedoes not substantially match the diameter of the hollow substrate tube,a mismatch between the hollow channels of the substrate tube and theinsertion tube give raise to such a turbulence. This is the case when aninsertion tube having an external diameter (and internal diameter)smaller that the internal diameter of the substrate tube is insertedinto one end of the substrate tube. This turbulence will lead to thedeposition of glass having a different composition that is more prone tocracking.

The second point being that the composition of the glass deposited atthe end of the discharge side of the hollow substrate tube differs fromthe glass in the rest of the substrate tube. It appears that thecomposition of the glassy deposition here is different compared to otherregions of the deposition area. Without wishing to be bound by anytheory, the inventors believe that the doping level is higher in theglassy deposition near the insertion tube, probably due to theturbulence which might lead to a locally decreased temperature at thedischarge side. It was found out that this glass is more prone tocracking.

The present invention provides a device that solves the above problems.The insertion tube is not inserted into the end of the substrate tubebut into the elongation tube.

Usually, the insertion tube is connected to a pump arranged for creatinga low pressure in the insertion tube such that the glass-forming gasesare sucked through the hollow substrate tube and the insertion tube.

In an even further embodiment, the elongation tube comprises a hollowinsertion tube having an internal diameter greater than or equal to aninternal diameter of the substrate tube.

The advantage of the use of an insertion tube is that no glass isdeposited on the interior of the elongation tube, such that the interiorof the elongation tube does need to be swiped clean when the internalvapour deposition process is finished. In such a case, only theinsertion tube needs to be removed.

In another embodiment, an end face of the hollow insertion tube islocated in the overlapping region, between the outer diameter of thehollow substrate tube and the inner diameter of the elongation tube.

In another embodiment, the hollow insertion tube is oriented and mountedin line with the hollow substrate tube such that a distance between thehollow insertion tube and the hollow substrate tube is at least 0.5millimeters, preferably at least 2 millimeters.

This embodiment provides the advantage that turbulence of theglass-forming gasses between the hollow substrate tube and the insertiontube is avoided, or at least reduced, as no mismatch between the twochannels of the hollow substrate tube and the insertion tube exists.

In yet another embodiment, the elongation tube is connected to the outersurface of the hollow substrate tube at an end face of the elongationtube.

The present invention relates, in a second aspect, to a method formanufacturing a precursor for a primary preform by means of an internalvapour deposition process, wherein glass-forming precursors are suppliedto a hollow substrate tube on a supply side thereof, which substratetube further has a discharge side, wherein an energy source is movedalong a length of the hollow substrate tube for the purpose ofgenerating deposition conditions in the hollow substrate tube, andwherein a plurality of glass layers is deposited on the inner surface ofsaid substrate tube to form said precursor for a primary preform,wherein an elongation tube is mounted over, and connection, in asubstantially air-tight manner, to the hollow substrate tube, at thedischarge side thereof, such that the hollow substrate tube extends intoan interior of the elongation tube, thereby defining an overlappingregion between the hollow substrate tube and the elongation tube, and inthat an internal diameter of the elongation tube at the overlappingregion is at least 0.5 millimeters larger than an external diameter ofthe hollow substrate tube at the overlapping region.

In this process a precursor for a primary preform is obtained or inother words a substrate tube having deposited layers of glass on itsinside surface thereof. After this process this precursor may besubjected to a collapsing process after which a primary preform may beobtained. Said primary preform may be overcladded or sleeved to obtain afinal preform that is used for drawing optical fibers.

During this process, preferably a plasma deposition process is used,e.g. PCVD.

The first step in this method is the provision of a hollow substratetube and the supply of glass-forming gasses into said hollow substratetube via the supply side thereof. An energy source is moved along alength of the hollow substrate tube for the purpose of generatingdeposition conditions or a so-called reaction zone in the hollowsubstrate tube, e.g. by the generation of a plasma. This ensures that aplurality of glass layers is deposited on the inner surface of saidsubstrate tube to form said precursor for a primary preform.

According to the present invention an elongation tube is mounted overthe hollow substrate tube, at the discharge side thereof, such that thehollow substrate tube extends into an interior of the elongation tube,and in that an internal diameter of the elongation tube is at least 0.5millimeters larger than an external diameter of the hollow substratetube.

In other words, the end face of the substrate tube is not in physicalconnection with the inner surface of the elongation tube. The advantagesthereof have been discussed above for the first aspect.

In an embodiment of the method, the elongation tube is fused to thehollow substrate tube.

In an embodiment of the method, the hollow substrate tube extends intothe interior of the elongation tube with a length, i.e. defining theoverlapping region, between 0.5 and 10 centimeters, preferably between 2and 5 centimeters, even more preferably between 2.5 and 3.5 centimeters.

In a further embodiment of the method the internal diameter of theelongation tube at the overlapping region is between 0.5 and 5millimeters, preferably between 1-2 millimeters, larger than theexternal diameter of the hollow substrate tube at the overlappingregion.

In another embodiment of the method, the elongation tube comprises ahollow insertion tube having an internal diameter greater than or equalto an internal diameter of the substrate tube.

In another embodiment of the method, an end face of the hollow insertiontube is located in the overlapping region, between the outer diameter ofthe hollow substrate tube and the inner diameter of the elongation tube.

In yet another embodiment of the method the hollow insertion tube isoriented and mounted in line with the hollow substrate tube such that adistance between the hollow insertion tube and the hollow substrate tubeis at least 0.5 millimeters, preferably at least 2 millimeters.

In an even further embodiment of the method the elongation tube isconnected to the outer surface of the hollow substrate tube at one endface of the elongation tube.

The elongation tube may be connected, i.e. fused, to the hollowsubstrate tube in a perpendicular manner, or under an angle.

Different aspects applicable to the examples of the methods according tothe present invention, including the advantages thereof, correspond tothe aspects applicable for the devices according to the presentinvention, as explained above.

The present invention relates, in a third aspect, to a substrate tubeassembly for use in an internal vapour deposition process, the substratetube assembly comprising a hollow substrate tube having a supply sideand a discharge side such that glass-forming precursors may be suppliedthrough the hollow substrate tube via the supply side thereof, whereinthe substrate tube assembly comprises an elongation tube mounted over,and connect, in a substantially air-tight manner to, the hollowsubstrate tube, at the discharge side thereof, such that the hollowsubstrate tube extends into an interior of the elongation tube, therebydefining an overlapping region between the hollow substrate tube and theelongation tube, and in that an internal diameter of the elongation tubeat the overlapping region is at least 0.5 millimeters larger than anexternal diameter of the hollow substrate tube at the overlappingregion.

In an embodiment hereof the elongation tube is fused to the hollowsubstrate tube.

In an embodiment thereof the hollow substrate tube extends into theinterior with a length between 0.5 and 10 centimeters, preferablybetween 2 and 5 centimeters; even more preferably between 2.5 and 3.5centimeters.

In another embodiment, the internal diameter of the elongation tube atthe overlapping region is between 0.5 and 5 millimeters, preferablybetween 1-2 millimeters, larger than the external diameter of the hollowsubstrate tube at the overlapping region.

FIG. 1 discloses a first example of a substrate tube assembly 1according to the present invention.

The substrate tube assembly 1 is to be used in an internal vapourdeposition process, such as a plasma vapour deposition process, whereinlayers of glass 7 are deposited on the interior of an hollow substratetube 8. During such a process, glass-forming precursors are suppliedthrough the hollow substrate tube 8, and at the same time an applicatoris used for generating electromagnetic radiation into a plasma that isgenerated inside the substrate tube 8.

FIG. 1 discloses the discharge side of the hollow substrate tube 8, i.e.the side at which the glass-forming precursors exit the hollow substratetube 8.

According to the present invention an elongation tube 3 is mounted overthe hollow substrate tube 8, at the discharge side of the hollowsubstrate tube 8, wherein the hollow substrate tube 8 extend into aninterior of the elongation tube 3. Further, the internal diameter 5 ofthe elongation tube at the overlapping region 2 is at least 0.5millimeters larger than an external diameter 4 of the hollow substratetube 8 at the overlapping region 2.

In the present situation the hollow substrate tube 8 extends into theinterior with a length 2 of approximately 3.5 centimeters. According toan embodiment of the present invention, the length at which the hollowsubstrate tube 8 extends into the elongation tube 3 should be in therange between 0.5 and 10 centimeters. This avoids mechanical stressbuild up at the connection point, i.e. the connection between the hollowsubstrate tube 8 and the elongation tube 3.

Further, the internal diameter 5 of the elongation tube, at least at theoverlapping region 2, is larger than the external diameter 4 of thehollow substrate tube 8. This provides the advantage that glassdeposited in the interior of the elongation tube 3 (not shown) is not incontact with the deposited glass 7 in the interior of the hollowsubstrate tube 8. As such any imperfections occurring the glassdeposited in the interior of the elongation tube 3 cannot propagate intothe deposited glass 7 in the hollow substrate tube 8.

The difference in the diameters 4, 5 should be chosen such that thespacing 6 between the hollow substrate tube 8 and the elongation tube 3is between 0.25 millimeters and 2.5 millimeters, preferably somewherebetween 0.5-1 millimeter.

FIG. 2 discloses a second example of a substrate tube assembly 11according to the present invention.

Parts or aspects in FIGS. 2 and 3 having the same reference numeral asin FIG. 1 indicate the same or similar parts or aspects.

According to the example shown in FIG. 2, an insertion tube 16 isplaced, i.e. mounted, within the elongation tube 15, which insertiontube is mounted in line with the hollow substrate tube 8. This meansthat a longitudinal axis of the hollow substrate tube 8 is aligned withthe longitudinal axis of the insertion tube 16. A distance 12 betweenthe hollow substrate tube 8 and the insertion tube 16 is advantageous,according to the present invention, as glass deposited in the interiorof the insertion tube 16 is not in contact with the deposited glass 7 inthe interior of the hollow substrate tube 8. As such, any inaccuraciesof the deposited glass in the insertion tube 16 cannot traverse towardsthe deposited glass in the interior of the hollow substrate tube 8.

In order to combat any turbulence of the glass-forming precursorsbetween the transition of the hollow substrate tube to the insertiontube 16, the internal diameter 14 of the insertion tube 16 should be atleast equal to the internal diameter of the hollow substrate tube 13. Inall embodiments of the present invention, not merely this embodiment ofFIG. 2, small tolerances of about 5% between these diameters areacceptable. In case the internal diameter 14 of the insertion tube issmaller than the internal diameter 13 of the hollow substrate tube 8, itis likely that turbulence occurs at the transition area between thesubstrate tube 8 and the insertion tube 15, resulting in a deposition ofan inferior quality of glass 7 at the discharge end of the hollowsubstrate tube 8.

FIG. 3 discloses a third example of a substrate tube assembly 21according to the present invention.

The difference between the substrate tube assembly 21 of FIG. 2 comparedto the substrate tube assembly 11 of FIG. 2 is that the hollow substratetube 8 also extend into the interior of the insertion tube 23. As longas the internal diameter 23 of the insertion tube 23 is larger than theinternal diameter of the hollow substrate tube 13, a spacing 22 betweenthe substrate tube 8 and the insertion tube 23 will be created. Thespacing 22 makes sure that there is no physical contact between theglass deposited in the insertion tube, i.e. residual soot, and glassdeposited 7 on the interior of the substrate tube 8.

The embodiments discussed above for any one of the aspects of theinvention are also applicable to the other aspects of the invention,unless stated otherwise.

The present invention is not limited to the embodiments as disclosedabove, and can be modified and enhanced by those skilled in the artbeyond the scope of the present invention as disclosed in the appendedclaims without having to apply inventive skills.

What is claimed is:
 1. A substrate tube assembly for use in an internalvapour deposition process, comprising: a hollow substrate tube having asupply side and a discharge side; and an elongation tube mounted over,and connected in a substantially air-tight manner to, the hollowsubstrate tube at the discharge side thereof; wherein the hollowsubstrate tube extends into an interior of the elongation tube, therebydefining an overlapping region between the hollow substrate tube and theelongation tube, and wherein an internal diameter of the elongation tubeat the overlapping region is at least 0.5 millimeters larger than anexternal diameter of the hollow substrate tube at the overlappingregion, and wherein the elongation tube further comprises a hollowinsertion tube positioned inside its inner diameter such that the hollowinsertion tube is aligned with and adjacent to the hollow substratetube, the hollow insertion tube having an internal diameter greater thanor equal to an internal diameter of the hollow substrate tube.
 2. Thesubstrate tube assembly according to claim 1, wherein the hollowsubstrate tube extends into the interior of the elongation tube, alength between 0.5 and 10 centimeters.
 3. The substrate tube assemblyaccording to claim 1, wherein the internal diameter of the elongationtube at the overlapping region is between 0.5 and 5 millimeters largerthan the external diameter of the hollow substrate tube at theoverlapping region.
 4. The substrate tube assembly according to claim 1,wherein the elongation tube is fused to the hollow substrate tube. 5.The substrate tube assembly according to claim 1, wherein the hollowsubstrate tube extends into the interior of the elongation tube a lengthbetween 2 and 5 centimeters.
 6. The substrate tube assembly according toclaim 1, wherein the hollow substrate tube extends into the interior ofthe elongation tube a length between 2.5 and 3.5 centimeters.
 7. Thesubstrate tube assembly according to claim 1, wherein the internaldiameter of the elongation tube at the overlapping region is between 1and 2 millimeters larger than the external diameter of the hollowsubstrate tube at said overlapping region.
 8. The substrate tubeassembly according to claim 1, wherein an end face of the hollowinsertion tube is located in the overlapping region, between the outerdiameter of the hollow substrate tube and the inner diameter of theelongation tube.
 9. The substrate tube assembly according to claim 1,wherein the hollow insertion tube is oriented and mounted in line withthe hollow substrate tube such that a distance between one end of thehollow insertion tube and one end of the hollow substrate tube thatextends into the interior of the elongation tube is at least 0.5millimeters.
 10. The substrate tube assembly according to claim 1,wherein the hollow insertion tube is oriented and mounted in line withthe hollow substrate tube such that a distance between one end of thehollow insertion tube and one end of the hollow substrate tube thatextends into the interior of the elongation tube is at least 2millimeters.
 11. The substrate tube assembly according to claim 1,wherein the elongation tube is connected to the outer surface of thehollow substrate tube at one end face of the elongation tube.